1. Field of the Invention
The present invention pertains to a storage system and a data migration method for a storage system.
2. Description of the Related Art
A storage system uses a memory device such as a hard disk drive and creates a memory area based on RAID (Redundant Array of Independent Disks). This physical memory area (logical volume), for example, is provided to a host computer (hereinafter referred to as a host) such as a server machine. The application program operating on the host provides information processing service to the client terminal connected to the host by writing data in the logical volume or reading data from the logical volume.
The data volume to be stored in the storage system is increasing day by day, and, depending on the type of data, there are cases where it is mandatory to store such data for a long period of time. Thus, in order to deal with the increasing demand, a new, high-capacity storage device is added.
When a high-capacity storage device is added to the storage system, data stored in the old, low-capacity storage device is migrated to the new, high-capacity storage device. As a method of migrating data from a certain storage device to another storage device, a method has been proposed for migrating data in a so-called online state without disrupting the service to the host (Japanese Patent Laid-Open Publication No. 2000-187608).
With a conventional storage system, the access destination of the host is switched to the storage device of the migration destination, and data migration is implemented between the storage devices. During data migration, the host accesses only the storage device of the migration destination to read and write the intended data. When data requested to be read from the host has already been copied to the storage device of the migration destination, the storage device of the migration destination will provide the data that it stores to the host.
Meanwhile, when data requested to be read from the host has not yet been copied to the storage device of the migration destination, the storage device of the migration destination read data from the storage device of the migration source and copies this in its own volume, and provides the acquired data to the host.
Therefore, when access is made to data not yet copied to the storage device of the migration destination, I/O (Input/Output) will be performed twice; that is, (i) data copy from the storage device of the migration source to the storage device of the migration destination, and (ii) data transmission from the storage device of the migration destination to the host. Thus, much time is required for providing the non-migrated data to the host, and the response of the storage system will deteriorate. Further, when the data migration between storage devices and the data communication between the storage device of the migration destination and the host respectively use the same communication network, wasteful data will flow on the network due to the I/O operation being conducted twice, and the network traffic will increase.
In particular, for instance, when the host is to distribute continuous data such as video data, the influence resulting from the generation of wasteful I/O operation will be significant. When a series of data groups is to be read out as with video data, if this series of data groups has not been copied to the storage device of the migration destination, the foregoing two I/O operations will be performed continuously. Therefore, the response of the storage device will deteriorate, the response of the service to be provided from the host to the client terminal will also deteriorate, and the network traffic will also increase.